A universal intracellular factor, the "M phase-promoting factor" (MPF), triggers the G2/M transition of the cell cycle in all organisms. In late G2, it is present as an inactive complex of tyrosine-phosphorylated p34.sup.cdc2 and unphosphorylated cyclin B.sup.cdc13. In M phase, its activation as an active MPF displaying histone H1 kinase activity originates from the specific tyrosine dephosphorylation of the p34.sup.cdc2 subunit by the tyrosine phosphatase p80.sup.cdc25. Little is known about the signals which control or determine timing of MPF activation and entry into mitosis or about ways in which those signals can be blocked or enhanced, resulting in inhibition or facilitation of entry into mitosis. A means of identifying agents which do so would be useful, particularly because it would provide a way of controlling mitosis